Cationic charge and polyspecificity of an integrin domain regulates infectivity of malaria parasites

Abstract

Cell-cell and cell-substrate adhesion is critical for many functions in life. In eukaryotes, I-domains mediate functions as divergent as tissue traversal by malaria-causing Plasmodium parasites as well as cell adhesion and migration by human leucocytes. The Idomain containing protein TRAP is important for Plasmodium sporozoite motility and invasion. Here we show that the I-domain of TRAP is required to mediate adhesional properties which can be partially preserved when the native I-domain is replaced by Idomains from human integrins or from an apicomplexan parasite that does not infect insects.By putting in vivo data and structural features in perspective we conclude that polyspecificity and positive charge around the ligand binding site of the I-domain are important for TRAP function. Our data suggest a highly preserved functionality of I-domains across eukaryotic evolution that is used by apicomplexan parasites to invade a broad range of tissues in a variety of hosts. the domain (Figure 1A compared to 1B and Figure 1D compared to 1E). The distance pistoned is equivalent to two turns of an α-helix. Uniquely in the apicomplexan I-domains, a segment N-terminal to the I-domain is disulfide linked to the last helical turn of the α7-helix in its closed conformation (Figure 1A-C). As this segment pistons out of contact with the remainder of the I-domain in the open conformation, the last two turns of the α7-helix with its cysteine unwind, the N-terminal segment with its cysteine moves in a similar direction, and these segments reshape to form a β-ribbon (Figure 1A). Because of close structural homology between human integrin αI-and apicomplexan adhesin I-domains in the regions shown in green in Figure 1A-E, we were able to engineer exchanges between them in this study. P. berghei parasites expressing TRAP without an I-domain show the I-domain is essential for motility and invasion. Parasites expressing TRAP with I-domains from other organisms including humans can have nearly intact motility and invasion. However, a poly-specific, cationic integrin αI-domain functions much better than a more specific, anionic integrin αIdomain. Together with altering the charge of the native TRAP adhesin, our results suggest that cationic surface charge around the MIDAS motif can enable polyspecificity and permit TRAP to function as an adhesin with diverse ligands in both vertebrate and arthropod hosts.